Vaccines are widely used to prevent disease and to treat established diseases (therapeutic vaccines). There remains, however, an urgent need to develop safe and effective vaccines and adjuvants for a variety of diseases, including those due to infection by pathogenic agents, cancers and other disorders amenable to treatment by elicitation of an immune response.
Protein antigens (e.g. subunit vaccines, the development of which was made possible by recombinant DNA technology), when administered without adjuvants, induce weak humoral (antibody) immunity and have therefore been disappointing to date as they exhibit only limited immunogenicity. An additional disadvantage of subunit vaccines, as well as of killed virus and recombinant live virus vaccines, is that while they appear to stimulate a strong humoral immune response when administered with adjuvants, they fail to elicit protective cellular immunity. Adjuvants are used experimentally to stimulate potent immune responses in mice, and are desirable for use in human vaccines, but few are approved for human use. Indeed, the only adjuvants approved for use in the United States are the aluminum salts, aluminum hydroxide and aluminum phosphate, neither of which stimulates cell-mediated immunity. Aluminum salt formulations cannot be frozen or lyophilized, and such adjuvants are not effective with all antigens. Moreover, most adjuvants do not lead to induction of cytotoxic T lymphocytes (CTL). CTL are needed to kill cells that are synthesizing aberrant proteins including viral proteins and mutated “self” proteins. Vaccines that stimulate CTL are being intensely studied for use against many viruses (e.g., HIV, HCV, HPV, HSV, CMV, EBV), intracellular bacteria (e.g., tuberculosis); intracellular parasites (e.g., malaria, leishmaniasis, shistosomiasis, leprosy), and all cancers (e.g., melanoma, prostate, ovarian, etc.). Thus adjuvants are needed that stimulate CTL and cell-mediated immunity in general.
Yeast have been used in the production of subunit protein vaccines, including those tested in the aforementioned HIV vaccine trials and the currently licensed hepatitis B vaccine; however, in this case yeast are used to produce the protein, but the yeast cells or subcellular fractions thereof are not actually delivered to the patient. Yeast have also been fed to animals prior to immunization to try to prime the immune response in a non-specific manner (i.e., to stimulate phagocytosis as well as the production of complement and interferon). The results have been ambiguous, and such protocols have not generated protective cellular immunity; see, for example, Fattal-German et al., 1992, Dev. Biol. Stand. 77, 115–120; Bizzini et al., 1990, FEMS Microbiol. Immunol. 2, 155–167.
U.S. Pat. No. 5,830,463, issued Nov. 3, 1998, to Duke et al. described the use of nonpathogenic yeast carrying at least one compound capable of modulating an immune response, and demonstrated that such complexes are efficacious at stimulating cell-mediated, as well as humoral, immunity. In particular, U.S. Pat. No. 5,830,463 demonstrated that yeast which are genetically engineered to express a heterologous antigen can elicit both a cell-mediated and a humoral immune response when administered to a mammal.
There is currently a need for improved vaccines that stimulate T cell-, and particularly cytotoxic T lymphocyte (CTL)-, mediated immunity against cell-associated or endogenous antigens. Targets for these vaccines include cells infected with viruses, intracellular bacteria and parasites, as well as cancers. The initiation of CTL-mediated immunity requires that antigenic peptides be presented in association with major histocompatibility (MHC) class I molecules on the surface of professional antigen presenting cells (APCs) and, in particular, dendritic cells (Dcs) (Ridge et al., Nature 393:474–8 (1998)). Dendritic cells are the major antigen presenting cells (APCs) for initiation of immune responses. As DCs are unique in their ability to activate naive CD4+ and CD8+ T cells, they play a crucial role in priming both MHC class II- and class I-restricted, antigen-specific T cell responses (Ridge et al., Nature 393:474–8 (1998); Banchereau et al., Nature 392:245–52 (1998)). However, exogenously introduced antigens, for example, those found in vaccines consisting of antigenic proteins or killed pathogens, are predominantly processed via the MHC class II pathway for presentation to CD4+ T cells (Moore et al., Cell 54:777–85 (1988)). These types of vaccines stimulate potent humoral immunity but are relatively ineffective at stimulating CD8+ CTL. This shortcoming has led to an investigation of vaccine strategies that specifically target DCs to present antigens via MHC class I in addition to class II. DCs have been shown to possess a unique pathway for processing exogenous antigen, especially in particulate form, for presentation by the MHC class I pathway (Rodriguez et al., Nat Cell Biol 1:362–368 (1999)). In this regard, various liposome-like, particulate preparations composed of antigenic proteins or peptides with added adjuvants have shown promise at stimulating CTL (Falo et al., Nat Med 1:649–53 (1995); O'Hagan, J Pharm Pharmacol 50:1–10 (1998)). The particulate nature of these immunostimulatory complexes (ISCOMS) allows them to be readily phagocytosed by DCs that are recruited to the site of vaccination and which become activated by the adjuvant moiety.
Dendritic cell-based cancer vaccines are under intense study as well. Dendritic cells are expanded ex vivo and “loaded” with peptides (derived from suspected tumor antigens) or mixed with the patient's cancer cells (DCs phagocytose the cells and present tumor antigens). However, peptide loading is inefficient and requires knowledge of the particular peptide being used. Phagocytosis of dead tumor cells and debris leads to class II MHC presentation but not class I MHC presentation which is needed for activation of CTL. Therefore, there is a need in the art for improved vaccines, including improved DC vaccines.